This invention relates to an imaging modality displaying signals containing both velocity and energy information derived from echoes of ultrasound signals from fluid flow or tissue motion, where the signals are represented by display features obtained from the energy and velocity information according to two-dimensional display feature maps. As used in the description herein below, the term "velocity" means the mean velocity and the terms "velocity" and "mean velocity" are used interchangeably, such as in the case of the velocity (i.e., mean velocity) of fluid flow and tissue motion.
Conventional Methods
Color Doppler imaging has been in use for more than a decade. The conventional color Doppler modalities are briefly described as follows:
I. Color Doppler velocity imaging
This is the most common color Doppler imaging mode where only the velocity component of the received Doppler signal is shown. FIG. 1A is a typical color map which is used to show flow velocities and directions. The upper and lower color bars in FIG. 1A may be composed of varying intensities and hues of color to show different velocity flow components. The upper and lower bars may be constructed from different color combinations to distinguish positive and negative flows. A baseline with no color is usually included to inhibit the representation of the lowest velocity flow states where the ultrasound system is not as reliable in detecting directional flows, or to remove stationary clutter signals.
II. Color Doppler Velocity and Variance Imaging
In this imaging mode, both the variance and velocity components of the received Doppler signal are estimated. The color map for color Doppler velocity and variance imaging is similar to the one in FIG. 1A, except that the top and bottom right corners of the color map are used to show flow variance while flow velocity is color-coded using the rest of the color map. This mode is especially useful for illustrating turbulent Doppler flow since flow turbulence is usually characterized by high flow velocities and variances.
III. Color Doppler Energy Imaging
Color Doppler energy imaging is recently recognized as an important color Doppler mode for perfusion imaging. In this mode, only the energy (or squared modulus) component of a received Doppler signal is shown. While the rotating phase of a Doppler signal is used to estimate its velocity and variance components, the squared modulus of the same signal is used for calculating the signal energy or power. Since phase detection is less accurate than square modulus detection especially in the case where signal-to-noise ratios (SNR) are low, the same Doppler system provides more sensitivity in energy imaging compared to velocity and variance imaging. Color Doppler energy imaging thus becomes a dominant mode for perfusion imaging where the perfusion signal is usually weak and may easily be submerged by noise. FIG. 1B is a typical color map for energy imaging in which only the energy components are color-coded.
IV. Color Doppler Energy and Velocity Imaging
In the early days of medical ultrasound imaging, people had unsuccessfully attempted a combined energy and velocity imaging mode. In this conventional combined mode, typically the top and bottom right comers of the color map are used to show flow energy while flow velocity is color-coded using the rest of the color map. This mode is not useful for perfusion imaging because only the high energy levels are shown; the low energy levels (perfusion signals) are not color-coded. While this mode was available on early color Doppler imaging systems, it was not adopted by clinical users and has since been removed from most, if not all, of the current color Doppler systems.
Disadvantages of the Above Conventional Methods
One clinical objective of this disclosure is to provide a color Doppler imaging mode which is capable of tissue perfusion imaging and providing flow velocities and directions at the same time. From the discussion in the previous section, it is clear that neither color Doppler velocity nor velocity/variance imaging can provide the necessary sensitivity desired for tissue perfusion imaging. Similarly, the above-described conventional color Doppler energy and velocity imaging mode is unsuitable for perfusion imaging.
Although color Doppler energy imaging provides the desired sensitivity, it is unable to distinguish flow directions and velocities. For example, in the diagnosis of liver cirrhosis, it is clinically significant to be able to observe liver perfusion in the tissue and flow directions in the larger blood vessels simultaneously.
From the above, none of the conventional methods is entirely satisfactory. It is therefore desirable to provide a new and improved imaging modality with improved information display capabilities.